Facebook Graph API - What is a Check In?

The documentation for an Open Graph Check-in 开发者_开发问答is available at: http://developers.facebook.com/docs/reference/api/checkin/ . I don't understand what it is. What is it?

Checking In, means when you (and your friends) are at location you can 'Check In' to that location by using facebook's list of locations. You can add a location if its not listed, and even add information to those locations. It shows up on your Wall that you 'Checked In' into 'Location X' with 'Friends Y and Z'

This would allow you to grab the users check in's perhaps to see popular locations they like, or determine other things that may be helpful from that information.